Conventional dispensing systems for selectively dispensing metered doses generally include a metering chamber which holds a measured amount of active ingredient to be dispensed at a later time. If the dispensing system results in charging the metering chamber upon completion of a dispensing process, the charge will remain in the metering chamber until the next dispensing process. Where the time between dispensing consecutive doses is relatively long, phase separation (partition) will occur, resulting in settling of the active ingredient in a non-uniform fashion inside the metering chamber. This problem occurs whether the active ingredient is in an emulsion or in a suspension. In all cases, even for solutions, an air vapor pocket is present as a result of the conventional pressure process for filling the dispensing apparatus. Some of the air pocket can enter the metering chamber along with a charge, thereby reducing the amount of active agent in the metering chamber.
Conventional dispensing systems include a spring-biased stem positioned in a metering chamber for providing a conduit through which a dose is dispensed. This is often disadvantageous when the spring is inside the metering chamber, because the active agent is likely to adhere to the spring and adversely affect the quantity of active agent that is actually dispensed. This has a deleterious effect upon the functioning of dispensing apparatus for delivering consistently uniform doses of material.
In conventional drug dispensers having a filling port or bypass conduit connecting the metering chamber to the main reservoir, the active agent may migrate between dispensing operation. If this occurs, the amount of active agent present in the chamber will be non-uniform from one dispensing operation to another; this helps to explain the dose to dose variability experienced with existing dispensers.
In order to combat the non-uniformity of dosing, many conventional dispensing devices are unnecessarily large and produce an unpleasant cold sensation when used for nasal drug administration. The cold sensation results from an undesirably large amount of propellant, e.g., Freon, which must be present in the metering chamber for dispensing a predetermined amount of drug or other material. In addition, the container must be significantly larger, increasing material and packaging costs.
It is, therefore, an object of the present invention to provide a new and improved metered aerosol dispenser and method of using such dispenser which overcome all of the above-stated disadvantages of conventional drug dispensers, and which provide a simpler construction and mode of operation than known dispensers.